Understanding the network attribute
Network attributes are properties of the network elements that control traversability over the network. Examples of attributes include the time to travel a given length of road, which streets are restricted for which vehicles, the speeds along a given road, and which are one-way streets.
Network attributes have five basic properties: name, usage type, units, data type, and use by default. Additionally, they have a set of assignments defining the values for the elements:
- The usage type specifies how the attribute will be used during analysis, which is identified as either a cost, descriptor, restriction, or hierarchy.
- Units of a cost attribute are either distance or time units (for example, centimeters, meters, miles, minutes, and seconds).
Descriptors, hierarchies, and restrictions have unknown units.
- Data types can be either Boolean, integer, float, or double.
Cost attributes cannot be a Boolean data type. Restrictions are always Boolean, whereas a hierarchy is always an integer.
- Use by default will automatically set those attributes on a newly created network analysis layer.
If a cost, restriction, or hierarchy attribute is set to be used by default, the network analysis layers that are created on the network dataset will be set to automatically use these attributes. Only one cost attribute in the network dataset can be set to be used by default. Descriptor attributes cannot be used by default.
Network attributes are created either in the New Network Dataset wizard (when defining a new network) or on the Network Dataset Properties dialog box on the Attributes tab. To create network attributes, first define the name of the attribute and its usage, units, and data type. Next, assign evaluators for each source that will provide the values for the network attribute when the network dataset is built. This is done by selecting the attribute and clicking Evaluators.
Network attributes can also have parameters that can be used by their evaluators. Parameters allow dynamic analysis with network attributes by modeling such descriptor attributes as truck height or weight, weather factors, or current speeds.
Certain attributes are used to measure and model impedances, such as travel time (transit time on a street) or demand (the volume of garbage picked up on a street). These attributes are apportionable along an edge; that is, they are divided proportionately along the length of an edge. For example, if travel time is modeled as a cost attribute, then traversing half an edge will take half the time as traversing the whole edge. If the travel time to traverse the edge is 3 minutes, it takes 1.5 minutes to traverse half the edge. If you are looking for a 1.5-minute route along this edge, the route feature will be created from the first half of the edge feature.
Network analysis often involves the minimization of a cost (also known as impedance) during the calculation of a path (also known as finding the best route). Common examples include finding the fastest route (minimizing travel time) or the shortest route (minimizing distance). Travel time (drive time, pedestrian time) and distance (meters) are also cost attributes of the network dataset.
The ArcGIS Network Analyst solvers do not support minimizing negative impedance values. If a cost attribute with negative values is used as the impedance attribute of a solver, it will treat those network elements with negative values as restricted.
Descriptors are attributes that describe characteristics of the network or its elements. Unlike costs, descriptors are not apportioned. This means that the value does not depend on the length of the edge element. For example, the number of lanes is an example of a descriptor on a street network. Speed limit of streets is another descriptor attribute for a street network. Although it is not a cost attribute and cannot be used as an impedance, it can be used in conjunction with distance to create a cost attribute (for example, drive time) that can be used as an impedance.
Restrictions can be identified for particular elements, such that during an analysis, restricted elements cannot be traversed. For example, one-way streets can be modeled with a restriction attribute so they can only be traversed from one end to another and not in the reverse direction. In all cases, a restriction attribute is defined using a Boolean data type.
In another example, certain sources where pedestrians are not allowed could be restricted using the attribute No_Pedestrians. In this case, the restriction can be used as a parameter during best route analysis to ensure the pedestrian does not use streets that are restricted.
Hierarchy is the order or grade assigned to network elements. A street network might have a road class hierarchy for separating interstates from local roads. In finding a shortest path from one point to another, the user preference to take or avoid interstates can be modeled through a hierarchy.
In ArcGIS Network Analyst, different classes of hierarchy can be grouped into three ranges: primary roads, secondary roads, and local roads. If your network has more than three classes of hierarchy, you can reclassify them into the supported ranges when you create your network dataset.
When you use a network that supports hierarchy for analysis, you can choose to create a route that uses hierarchy or you can create an exact route not using hierarchy.